Protein phosphorylation is one of the most common and extensively studied posttranslational modifications. Relatively little is known, however, about phosphorylation of histidine residues, which occurs at the imidazole nitrogens. While some anti-phosphohistidine antibodies have been described, they can cross-react with antibodies to other phosphorylated amino acids and/or do not bind phosphohistidines with desired affinity.